Two main reasons....
1) To be able to adjust the shutter speeds. Not quite the issue now as it was in the days of film. Film was rated at a certain ISO, and you got the best results (with some exceptions of course) if you shot at or very near that ISO. It was possible to shoot film at a different ISO by altering the developing, but once you had started to shoot it became very very difficult to change the ISO since that would mean somehow physically separating the low ISO end of the film from the high ISO end the film so that each end could be processed individually. I had to learn to how to do this, but thankfully never actually needed to. With digital cameras you can change ISO from shot to shot order to change the shutter speed (within limits of course) while keeping the f/stop constant (more or less). Back then you needed to adjust your f/stop in lock-step with the shutter speeds.
2) DoF. Traditionally portraits are taken with a shorter DoF and landscapes with a longer DoF. It's not unusual for landscape photographers with medium or large format systems to shoot at f/32, f/45, or even f/64+ in order to preserve the detail from the foreground to the back. In the mid-1900s there was a group of photographers who formed the f/64 Group because they insisted the only good landscape was one shot at about f/64 or higher (OK, they didn't actually say that .... but they did believe that a landscape should be in focus from back to front, and that could only be achieved by shooting at very small apertures.) Ansel Adams, Imogen Cunningham (iirc) and one or two of the Westons formed part of the f/64 group. Plus some others I should know, but can't quite pull out of the memory. But you can Google them if you want to know more.
Knowing how film cameras worked, and how film worked, helps to explain the "why" of many digital camera functions. Many of the functions today are what they are simply because they are descended from how film cameras needed things to work.
Hope this helps....